The translation of Mushin is "no mind". This is a mental state in which the thoughts of the karateka are clear and focused. During combat Mushin allows for complete concentration on one's actions and reactions without any distractions of thoughts in the mind that can lead to momentary lapses in one's defense. For example, during combat, if one is actively thinking about what technique one is going to execute or trying to anticipate what technique the opponent is going to use, there are momentary lapses in ones concentration and this can lead to "mental openings" in one's defense. These openings leave one exposed for momentary periods of time when one is more vulnerable to attack. Worse is to be thinking of a thought unrelated to the combat or match currently engaged in. During combat it is still necessary to think of technique and be analyzing one's opponent but these thoughts need to be felt rather than having concentrated thought directed to them.
The trained karateka can feel the openings in the opponent and automatically knows the strengths and weakness of the opponent without actual physical interaction.
Utilizing mushin the karateka's mind is in a constant state of readiness. Distracting thoughts are blocked out of one's mind and only attention to one's opponent is the focus of the mind. Active thoughts are kept to a minimal only momentarily being utilized to analyze the opponent when necessary or when one cannot "feel" the opponent's strengths and weakness. Once the analysis is quickly made one must immediately transfer back to the mushin state of mind.
The state of mushin allows for complete unity of the mind and body and eliminates any interference from mental distractions, both internal and external. By internal I mean one's own thoughts. By external, I mean by distractions that are occurring around one's self, which can cause interference in one thoughts and cause lapse's in one's defense.
Mushin is also useful in the performance of kata. This is especially true in the blocking of external interference that is the commonplace in tournament karate. During a kata performance in competitive karate there are usually several rings working independently with the crowd reacting to the action in surrounding rings. If the competitor allows one to become distracted with the external interference, concentration is lost. Mushin also allows one to block the stress caused by performance of kata in front of an audience. With mushin, the crowd is blocked out of the mind with 100 percent of one's concentration focused on the actual movements of the kata. For a karateka to be effective at utilizing mushin in kata, the kata must be practiced to the degree where it becomes second nature and active thoughts as to what move comes next is allowed to be eliminated from the thought process. If the next move must be constantly thought of, the state of mushin cannot be attained.
Mushin is mental concentration as it's highest level. Mind with no mind.